1. Field of the Invention
The present invention relates to a heterocyclic compound. Also, the present invention relates to a light-emitting element, a light-emitting device, an electronic device, and a lighting device using the heterocyclic compound.
2. Description of the Related Art
In recent years, research and development have been extensively conducted on light-emitting elements using electroluminescence (EL). In the basic structure of such a light-emitting element, a layer which contains a light-emitting substance is interposed between a pair of electrodes. By voltage application to this element, light emission from the light-emitting substance can be obtained.
Since such light-emitting elements are self-luminous elements, they have advantages over liquid crystal displays in having high pixel visibility and eliminating the need for a backlight, for example, thereby being considered as suitable for flat panel display elements. Light-emitting elements are also highly advantageous in that they can be thin and lightweight. Furthermore, very high speed response is one of the features of such elements.
Furthermore, since such light-emitting elements can be formed in a film form, they make it easy to provide planar light emission, thereby achieving large-area elements utilizing planar light emission. This is difficult to obtain with point light sources typified by incandescent lamps and LEDs or linear light sources typified by fluorescent lamps. Thus, light-emitting elements have great potential as surface light sources applicable to lightings and the like.
A light-emitting element utilizing EL is driven by injection of electrons from a cathode and holes from an anode into a layer containing a light-emitting substance which is interposed between a pair of electrodes. The electrons injected from the cathode and the holes injected from the anode recombine in the layer containing the light-emitting substance to form molecular excitons. The molecular excitons release energy in returning to a ground state. In the case where the energy is released as light having a wavelength corresponding to that of visible light, light emission can be seen. Excited states of organic compounds can be a singlet state and a triplet state, and light emission can occur either of the excited state.
The emission wavelength of a light-emitting element is determined by the difference of energy between the ground state and the excited state, that is, an energy gap. Therefore, by appropriate selection or modification of a structure of the molecule that contributes to light emission, any color of light can be obtained. When a light-emitting device is fabricated using light-emitting elements capable of emitting light of red, blue, and green, which are the three primary colors of light, the light-emitting device can perform full color display.
Manufacture of high performance full-color light-emitting devices needs red, blue, and green light-emitting elements which are excellent in lifetime, emission efficiency, and the like. The recent development of materials has achieved good characteristics of red and green light-emitting elements. However, as for blue light-emitting elements, sufficient characteristics have not been obtained. For example, Patent Documents 1 and 2 reported a light-emitting element having relatively high emission efficiency. However, in order to realize high performance full-color displays, further higher emission efficiency have been required.